Conventional advertising signs usually feature variously shaped luminous hollow tubing filled with an excitable gas such as neon. The letters or design of the sign are formed by a plurality of adjacently or serially mounted sections or lengths of the neon tubing. Each such section usually comprises a pre-curved or bent tubing length requiring two or more support means when it is installed in its display environment, such as a building front. It has been awkward at least, to readily affix this sign tubing to existing mounting or support devices, especially when it will be appreciated that most installations involve elevations well above the ground with the workmen operating from ladders or scaffolding and frequently mounting the tubing within shadow boxes suspended in a vertical plane. Additionally, a problem has existed in providing convenient means to secure the running lengths of high voltage electrical cable leading to the ends of each tube length within a sign assembly. Furthermore, the need exists to provide a support device that will accommodate tubing of various diameters while allowing of the same positive clamping action, regardless of the size tubing involved.